During the manufacture of some shaped refractory products, such as brick, it is customary to impregnate the fired product with petroleum pitch or tar or other resinous material in order to provide a refractory which, after tempering or coking, has a lower porosity than would otherwise be the case. The tar impregnation improves the resistance of the refractory to penetration by gases, liquids, and other materials which are likely to be encountered by the refractory under service conditions. In the case of refractories to be used in the basic metal industries, for example as furnace linings, the impregnated refractory is more non-wetting to liquid metals and slags.
Conventionally, tar impregnated products, hereinafter referred to for convenience as brick, have been shipped to the user after impregnation and installed to provide a brick-lined piece of equipment. Thereafter, as the equipment is placed into service, the brick are heated to operating temperature resulting in the emission of large quantities of volatile constituents of the pitch or tar used as the impregnant. Because of environmental concerns about these emissions (which are primarily aromatic hydrocarbons) users of the brick are desirous of obtaining impregnated brick from which substantially all of the volatiles have been removed. Such brick are referred to herein as coked brick.
As is known to those skilled in the art, refractory products are sometimes subjected to a tempering process and sometimes to a coking process. The fundamental difference between the two processes is that tempering process is conducted at about 400.degree.-600.degree. F. whereas the coking process is conducted at above 1000.degree. F. As a result of these differences in temperature, tempering serves to remove some of the volatiles present in the impregnant. Coking, on the other hand, eliminates essentially all of the volatiles. Due to the higher temperatures employed in coking more sophisticated designs of the coking furnace are required in order to protect the ware being coked.
It may seem to be a relative easy task for the brick supplier to preheat or coke impregnated brick prior to shipment to customers in order to remove the volatiles. However, the solution to the problem is not that simple since the tar impregnation process is typically carried out on a large volume of tightly hacked brick. Thus a typical hack would occupy a space of the order of 27.times.35.times.50 inches and contain 450 equivalent standard brick.
In theory, removal of volatiles from the brick could be accomplished by heating the hack in a furnace to a temperature sufficient to drive off the volatiles. Unfortunately, such a practice is unsatisfactory since the time required to heat the innermost brick in the hack to a temperature sufficient to cause essentially complete volatilization of the organic substances originating from decomposition and/or distillation of pitch fractions either as a gas or vapor would take so long that the non-volatile residue from the impregnant, which provides the desired properties of impregnated brick, would be completely burned away in the more exterior brick in the hack.
On the other hand, a hack of impregnated brick could be disassembled into individual brick or small groups of brick so as to minimize the time consumed in the volatilization process. That is also an undesirable solution since very significant handling costs would be incurred because the coked brick would have to be reassembled into large hacks for shipping purposes. A further alternative is to heat the hacks very slowly to the temperatures required for volatilization of the organics, hopefully avoiding burning the more exterior brick in the hack. But that approach, even if feasible, would add a very significant cost to the impregnated products.
The environmental problems due to the emission of volatile organics originating from pitches or tars is not a problem limited to the refractory industry. The problem has been addressed, for example, in the manufacture of carbon anodes intended to be used for the electrolytic reduction of aluminum. Thus in U.S. Pat. No. 4,569,835 to Di Cio et al, carbonaceous blocks are produced by pressure molding or extruding a carbon-carrying mixture, such as petroleum coke, anthracite, gas black, graphite and the like, with a binder material, such as tars and/or pitches. The blocks are then calcined in order to bake the blocks to achieve the desired mechanical and electrical properties. The calcining operation is conducted in a tunnel furnace, wherein the blocks are coated or covered on all sides with powdered carbon to protect them against oxidation and stored in muffles mounted on carriages which are pushed through the furnace in an oxidizing atmosphere in accordance with a preset thermal schedule. During heating, primarily in the temperature range of 200.degree.-600.degree. C., (about 400.degree.-1100.degree. F.) the blocks being calcined release volatile organics originating from the pitch or tar. Heating gradients on the order of 40.degree.-45.degree. C. per hour are used during this temperature range in order to avoid cracking the blocks. The blocks remain in the furnace for 160 hours and they ultimately reach temperatures of the order of 1000.degree.-1200.degree. C. (1800.degree.-2200.degree. F.).
The muffles used in the process disclosed by Di Cio et al are described as usually open at the top with the walls and the bottom made from a porous refractory material which is pervious to the volatiles from the binder. As a result, combustion of the volatiles takes place on the exterior of the muffle as well as on the surface of the free or open layer of protective or filler powder. Alternatively, a refractory material can be placed over the top of the muffle to maintain combustion of the carbonaceous powder at the lowest level.
To date however, the refractory industry has not developed a cost effective and environmentally acceptable process for the removal of volatile constituents contained in tar or pitch impregnated refractory products prior to the use of such products in refractory-lined equipment.